Search

Our knowledge about dense matter occurring in the cores of neutron stars remains limited, as those densities are beyond reach on Earth. Fortunately, the dense matter can be probed not only with astrophysical observations but also in terrestrial heavy-ion collision experiments. In this thesis, we developed a Bayesian inference method to combine data from astrophysical observations of neutron stars with gravitational waves, electromagnetic waves from radio to X-ray, and heavy-ion collisions of gold nuclei at relativistic energies, with information from microscopic nuclear theory calculations to improve our understanding of dense matter. This way we arrived at state-of-the-art constraints on the properties of supranuclear matter. Besides the nuclear physics community, the astrophysics community also benefits from an accurate understanding of neutron star matter. This thesis showcases two such applications. First, we have shown how one can distinguish a low-mass black hole from a neutron star when no light is observed. Secondly, we introduced methods to check if a binary neutron star merger signal is gravitationally lensed even if we only see one image. In addition, we developed methods to test the validity of general relativity. First, we devised tools for establishing the presence of polarizations beyond the ones of general relativity, with a limited number of detectors. Secondly, we demonstrated how one could distinguish an exotic compact object from a black hole by looking for the signatures of resonant excitations. Einstein's theory withstood all of our tests.

In this dissertation, we present a set of measurements of anisotropic flow in heavy-ion collisions, aimed at investigating the properties of hot and dense QCD matter. We analyse data collected by the ALICE experiment at the LHC between 2010 and 2015, colliding lead ions (208Pb) at a center-of-mass energy per nucleon of a few TeV. Anisotropic flow quantifies the degree of anisotropy in the azimuthal distribution of momenta of particles produced in the collision and provides information on collective dynamics of the system. In particular, we study how anisotropic flow, on average, depends on the collision energy of the ions, on how much they overlap, and on particle kinematic variables (e.g. velocity). We then analyse how anisotropic flow, in particular elliptic flow, changes between different collisions, which is mostly determined by the variations in the shape of the QCD system. We found all aforementioned results to be essentially compatible with expectations, thus confirming our current understanding of heavy-ion collisions. These results also constitute a large and comprehensive data set that will hopefully help to constrain even further a few important parameters in theoretical models, such as the specific shear viscosity and its temperature dependence. We then study the strong magnetic fields created in such collisions, which could provide information on so far unexplored features of QCD and on the electromagnetic response of the system. More precisely, we search for evidence of the Chiral Magnetic Effect, measuring again azimuthal distribution of particle momenta. We use simulations to understand the expected correlation between these magnetic fields and the shape of the QCD system. This helps us to decouple the contributions to anisotropic flow coming from magnetic fields and from other phenomena, unrelated to such fields. We conclude that no significant contribution of the Chiral Magnetic Effect is present in the azimuthal distribution of particle momenta and we quantify this statement with an upper limit. We also investigate another possible effect of the magnetic field, namely a difference in directed flow according to particle electric charge. Again, we find no significant evidence of such phenomenon, although results are suggestive; future work will provide a definitive answer. Finally, we conclude exploring the prospects for future measurements of anisotropic flow with the ALICE experiment. We also present the results of two exploratory studies and discuss their potential and limitations.

Two-particle angular correlations are presented from Pb-Pb and pp collisions at 2.76 TeV. These measurements are powerful tools to study jets in a transverse momentum regime where they cannot be reconstructed over the large fluctuating background produced in heavy-ion collisions. In these studies, the distribution of azimuthal angle (Δφ) and pseudorapidity (Δη) differences of particle pairs is calculated. Jets manifest themselves as a peak around (Δφ, Δη) = (0, 0) and as a structure elongated in Δη at Δφ = π. The jet peak is studied as a function of transverse momentum and centrality in Pb-Pb collisions, and it is compared to pp collisions and to results from A Multi-Phase Transport model (AMPT). In Pb-Pb collisions, an asymmetric broadening of the jet peak towards central collisions at low transverse momentum is found, and in addition an unexpected depletion around (Δφ, Δη) = (0, 0) is observed. The comparison to AMPT suggests that the two effects are connected to large radial and longitudinal flow, therefore they can arise from an interplay of the jets with the flowing medium produced in the collisions of heavy-ions. The measurements are done by the ALICE detector, which will be upgraded during the second long shutdown of the LHC in 2019-2020. During this upgrade, the Inner Tracking System of ALICE will be replaced by a detector consisting of seven concentric layers of Monolithic Active Pixel Sensors. The prototypes of this detector and the setup used for their characterization are discussed in details. Tests at test beam facilities are carried out on unirradiated sensors and irradiated sensors are tested as well to study the long term functionality of the detector. The results of these measurements are presented, and it is found that the prototypes fulfill the requirements of the upgrade in terms of detection efficiency, spatial resolution and noise occupancy both before and after irradiation, making them suitable for the operation in the environment in the ALICE experiment.

The knowledge of the gluon PDFs within hadrons is obtained indirectly from the results of Deep Inelastic Scattering experiments, which have a limited range towards small values of the momentum fraction x. At low x the behaviour of the PDFs is predicted by QCD evolution, which leads to a (unphysical) divergence of the gluon PDFs. In order to have a consistent description of hadrons, non-linear effects allowing for the gluon PDFs to saturate must occur. One of the proposed descriptions of the gluon fields in the saturation regime is the Colour Glass Condensate (CGC). Among the experimental observables that can help prove the validity of the CGC theory, the production of direct photons in hadronic collisions at large rapidity is one of the cleanest, but also most challenging observables. In fact, prompt photons are produced, in hadronic collisions, mainly via the qg-Compton scattering, which, at Leading Order, allows us to directly relate the energy and momentum of the final state photon to those of the initial state gluon. Photon measurements are particularly difficult because of the huge background from photons produced in the decay of short-living neutral hadrons such as the neutral pion. In this thesis, the cross section for isolated photons in pp collisions at √s = 7 TeV, produced at midrapity, is presented. The measurement is performed by analysing the data collected by the ALICE collaboration in 2011, with the help of a trigger algorithm that selects events with a high-energy deposit in the ALICE-EMCal. A clustering procedure allows us to reconstruct the total energy of the particle interacting with the electromagnetic calorimeter. After selecting single proton-proton collision, we restrict our analysis to clusters produced only by neutral particles. To estimate the contribution of background clusters (produced both by photons from neutral hadron decays and by photons from fragmentation processes) in our sample, a data-driven method is used. The applicability of this method has been tested on realistic simulated samples, which are used to correct also for inefficiencies due to detector effects. The systematic uncertainties of the measurement have been quantified and presented. The cross section for isolated photon production in pp collisions at √s = 7 TeV is then computed and compared to the state-of-the-art NLO pQCD calculations available. The measured cross section is found to agree well with the theoretical prediction. The result presented calls for a measurement of the same observable at large rapidity, where the onset of saturation effects is expected. For this purpose, the Forward Calorimeter has been proposed as an upgrade of the ALICE detector.

A prototype of a Si-W EM calorimeter was built with Monolithic Active Pixel Sensors as the active elements. With a pixelsize of 30 μm it allows digital calorimetry, i.e. the particles' energy is determined by counting pixels, not by measuring the energy deposited. Although of modest size, with a width of only four Moliere radii, it has 39 million pixels. We describe the construction and tuning of the prototype and present results from beam tests and compare them with predictions of GEANT-based Monte Carlo simulations. We show the shape of showers caused by electrons in unprecedented detail. Results for energy and position resolution will also be given.

The low-x structure of protons and in particular nuclei is not well constrained experimentally, while the knowledge about it is a crucial ingredient for the interpretation of many measurements at high-energy hadron colliders. It is widely expected that the growth of parton densities at low x predicted from linear QCD evolution cannot continue indefinitely, and that non-linear effects will lead to gluon saturation. Many experimental observations are compatible with saturation, however no real proof has been found yet. We propose the measurement of forward direct photons in proton-nucleus collisions at the LHC as a decisive probe of gluon saturation. Due to the harsh environment of such a measurement, existing detectors are not suitable. In particular an extremely high-granularity electromagnetic calorimeter is required, which we propose as a detector upgrade to the ALICE experiment, the Forward Calorimeter (FoCal). To facilitate the design of the upgrade and to perform generic R&D necessary for such a novel calorimeter, a compact full high-granularity electromagnetic calorimeter prototype has been built. This prototype is a Si/W sampling calorimeter using CMOS sensors of the MIMOSA type with a pixel pitch of 30 micron and binary readout with a total of ~39 million pixels. About 85% of the total volume is W absorber, a small Molière radius of 10.5 mm is expected. The raw data shows the properties and limitations of this prototype. To obtain a good performance, several selection procedures and an additional analysis methodology based on hit density is applied. In simulation, to make a valid comparison, misalignment, noise, and charge diffusion are considered. A realistic detector is simulated, a realistic number of noise pixels has also been added. To study the effects of dead areas, both the `ideal detector' and `real detector' are simulated. We present on performance studies of the prototype with test beams at DESY and CERN in a broad energy range. The results of the measurements demonstrate that a very small Molière radius and good linearity of the response. Unique results on the detailed lateral shower shape, which are crucial for the two-shower separation capabilities. The studies demonstrate the feasibility of this high-granularity technology for use in the proposed detector upgrade. Furthermore, they show the extremely high potential of this technology for future calorimeter development.

In ordinary matter, quarks and gluons are confined inside hadrons by the strong interaction. At extreme conditions of temperature and energy density, a new state of matter is formed, called quark-gluon plasma (QGP). This is made of deconfined quasi-free quarks and gluons. Based on the current cosmological picture, the quark-gluon plasma was the state of our universe few $\mu$s after the Big Bang. Moreover, there is evidence that a degenerate state of matter with similar properties to the QGP exists in the inner core of neutron stars and other compact astrophysical objects.Microscopic and extremely short-lived quantities of such a nuclear plasma can be created in ultra-relativistic heavy ion collisions. Its properties can be studied through several experimental probes using dedicated detectors installed around the collision region. This interesting branch of research is part of the experimental program of the Large Hadron Collider (LHC) at CERN, where lead ion beams are accelerated to unprecedented energies.The QGP properties, in principle, can be described by Quantum-Chromo Dynamics (QCD), the theory of the strong interaction. However, a description of the system based on QCD first principles is extremely complicated due to the relatively low energy scale involved (compared to $\Lambda_{QCD}$), which does not allow a perturbative approach. Further complications arise from many-body properties of QCD which are anyhow extremely interesting to explore.The deconfined medium created in heavy-ion collisions rapidly evolves, passing through several thermodynamic stages.Photons and dileptons are unique tools to study the properties of heavy-ion collisions. These particles are continuously emitted and they cross the medium with negligible interaction, thus carrying undisturbed information on their production source.Electromagnetic probes provide complementary information to hadronic probes, allowing to constrain the theoretical models used for the description of the system in the early stages. Thermal photons and dileptons carry information on the system temperature. Moreover, in-medium modifications of low-mass vector mesons spectral functions can be studied through their dilepton decay channels. These effects have since long been proposed as signatures of chiral symmetry restoration.Dileptons are also sensitive to heavy-flavor production, which gives a significant contribution to the intermediate mass region of the dilepton spectrum ($m_{\phi} < m_{l^{+}l^{-}} < m_{J /\psi}$).In this thesis, the dielectron production in Pb--Pb collisions at $\sqrt{s_{\mathrm{NN}}}=2.76\ \mathrm{TeV}$ with the ALICE experiment at the LHC has been studied. ALICE is the detector at the LHC dedicated to the study of heavy-ion collisions. The ALICE excellent tracking and particle identification capabilities make this experiment well suited for dielectron measurements. The main focus of this work has been the study of the low-mass region of the dielectron invariant mass spectrum, where contributions from thermal dileptons and from modified low-mass vector mesons are expected. The fraction of virtual direct photons has been measured, and the dielectron spectrum has been compared to the expected contributions from hadron decays, thermal dileptons and in-medium $\rho^{0}$ and $\omega$, resulting in good agreement within the experimental uncertainties.The future perspectives for the dielectron measurement and the predicted scenario after the ALICE upgrade are also presented.

The thesis presents the first measurements of the yields of jet-like structures at low transverse momentum in p-Pb collisions. A statistical method known as two-particle correlations is used to overcome the difficulties in identifying such structures with standard jet-reconstruction algorithms. Moreover, a subtraction procedure has been developed to disentangle the jet-like contribution from the long-range correlations structures (so-called 'double ridge') found in p-Pb collisions, which are attributed to collective phenomena.Through the study of jet-like structures, this method allows to estimate the number of independent sources of particle production, proportional to the number of multiparton interactions in PYTHIA simulations.

Jets are used to probe the quark-gluon plasma (QGP) that is created in heavy-ion collisions, by using the fact that medium-induced parton energy loss from elastic and radiative interactions between partons and the QGP lead to a modification of the measured jet spectrum. The dependence of the energy loss on the in-medium path-length provides insight into the energy-loss mechanisms and can be studied by measuring jet production relative to the orientation of the second-order symmetry plane. The azimuthal asymmetry in the jet production is quantified as `jet flow’, defined as the second-order coefficient of the Fourier expansion of the azimuthal distribution of jets relative the orientation of the symmetry axes of the initial nucleon distribution of the collision overlap region. In this dissertation, measurements of jet flow of R = 0.2 charged jets, reconstructed with the anti-kt jet finder algorithm in Pb-Pb collisions with 0-5% and 30-50% collision centrality are presented. Jets are reconstructed at mid-rapidity using charged constituent tracks with transverse momenta between 0.15 and 100 GeV/c, and are required to contain a charged hadron with a transverse momentum larger than 3 GeV/c. The underlying event energy is subtracted jet-by-jet using a description which takes into account dominant second and third hydrodynamic flow harmonics. Jet flow is obtained from jet yields measured as function of transverse momentum, with respect to the experimentally accessible event plane, which is reconstructed at forward rapidities. The reported jet flow has been corrected back to the azimuthal anisotropy with respect to the underlying symmetry plane by applying an event plane resolution correction. The jet spectra are corrected for fluctuations in the background transverse momentum density and detector effects through an unfolding procedure which is applied for different azimuthal orientations independently. The detector corrections correct back to particle level jets consisting of only primary charged particles from the collision. Significant non-zero jet flow is observed in peripheral collisions. The observed jet flow in central collisions is of similar magnitude, but the uncertainties are larger and therefore the results are not significantly different from zero. The azimuthal dependence of the jet production is similar to suppression observed in measurements of flow of single charged particles at high transverse momenta and flow of jets comprising both charged and neutral fragments. Good agreement between the data and predictions from JEWEL, an energy-loss model simulating parton shower evolution in the presence of a dense QCD medium, is found in peripheral collisions.

The thesis presents the results of the analysis of azimuthal angular correlations of D mesons and charged particles with the ALICE detector, in pp and p-Pb collisions at center-of mass energies of 7 and 5.02 TeV respectively. The measurements have been performed differentially as function of the transverse momentum (pT) of the D mesons as well as of charged particles. The comparison of the correlation distribution (and of the relative parameters) obtained from the measurements in the two collisional systems, provides insights into the effect of cold nuclear matter effects on the fragmentation process and, indirectly on the heavy-quark production mechanisms. In addition, both the measurements represent the baseline for future measurements in Pb-Pb collisions. The correlation distributions are compared also to predictions of different Monte Carlo models, namely different tunes of PYTHIA, and POWHEG. An outlook of the near-future measurements in Pb-Pb collisions as well as of measurements using a data sample with a trigger provided by the electromagnetic calorimeter is also presented in the thesis.

ALICE (A Large Ion Collider Experiment) is one of the four main experiments at the Large Hadron Collider (LHC) optimized for the study of heavy-ion collisions. The prime aim of the experiment is to study in detail the behaviour of nuclear matter at high densities and temperatures. In ALICE also proton-proton collisions are studied both as a reference for lead-lead collisions and in physics areas where ALICE is competitive with other LHC experiments. In order to characterise the system produced in heavy-ion collisions, it is necessary to use a wide variety of experimentally accessible observables, which can help to disentangle the different physical mechanisms that characterise the various collision stages. Among the different experimental observables, the focus of this thesis will be on anisotropic flow, one of the most important experimental observables used in heavy-ion collisions to study the properties of the QGP. For this purpose, a comprehensive study of the elliptic flow of hadrons, electrons from heavy-flavour hadron decays, and direct photons, all which are assumed to interact differently with medium, has been carried out. The v2 coefficient of low pT hadrons, mainly coming from the hadronization of soft partons within the bulk, carries direct information on the hydrodynamic properties of the partonic medium created in heavy-ion collisions. Among the different hadron species, the Φ-meson is of great interest because it is predicted to have a small hadronic cross section, and is therefore less affected by the interactions between hadrons during the hadronization phase of the collision. The v2 coefficient also allows to study the interaction strength of heavy-quarks with the expanding medium and their possible thermalisation in the medium itself. Due to their large masses, heavy quarks are produced at the initial stage of the collision, almost exclusively in hard partonic scattering processes. Therefore, they experience the full evolution of the system, propagating through the hot and dense medium and loosing energy via radiative and collisional scattering processes. Heavy-flavour hadrons and their decay products are thus effective probes to study the properties of the medium created in the heavy-ion collisions. Direct photons on the other hand, since they do not interact with the strongly-coupling medium created in these collisions, carry undistorted information about the system at their production time. The elliptic flow of direct photons strongly depends on the production mechanism. Small flow would be associated with early production while a large, hadron-like flow, would point towards late production in the medium.

Identified particle production ratios are important observables, used to constrain models of particle production in heavy-ion collisions. Measurements of the inclusive particle ratio in central heavy-ion collisions showed an increase of the baryon-to-meson ratio compared to proton-proton collisions at intermediate pT, the so-called baryon anomaly. One possible explanation of the baryon anomaly is that partons from the thermalized deconfined QCD matter hadronize in a different way compared to hadrons produced in a vacuum jet. In this work we extend on previous measurements by measuring particle ratios in the yield associated with a high-pT trigger particle. These measurements can potentially further constrain the models of particle production since they are sensitive to the difference between particles from a jet and particles that are produced in the bulk. We start by developing a particle identification method that uses both the specific energy loss of a particle and the time of flight. From there, we present a method to identify the particle yield associated with a high-pT trigger particle. We then measure the proton-to-pion and kaon-to-pion ratio in the jet peak and background of the associated yield in central Pb-Pb, central p-Pb and pp collisions. The observation that these particle ratios are higher in the background compared to the jet peak suggests that there is a difference between the mechanism of hadron production in the the underlying event and the jet. Furthermore, our results show that the particle ratios in the jet peak depend on the size of the system.

The subject of this thesis is a new type of electromagnetic calorimeter, which is not based on proportionality of deposited energy, but instead on counting the number of showering particles. This calorimeter is a proof of principle for a proposed upgrade of the ALICE experiment at the Large Hadron Collider. This thesis starts with giving the reader an understanding of the fundamental processes governing the development of electromagnetic showers, in order to correctly comprehend the effects underlying the performance of the calorimeter. It also aims to explain how the different choices in detector construction in combination with these processes affect the measurements of particles. Next, the choice of detector components is explained as well as the working of these components. This is followed by the explanation of the software used and the different steps in both data processing and analysis are examined piece by piece and should give a prospective user of this software an understanding of the underlying principles. This is followed by a discussion the simulation software used and the additions made to this software to fully model the behaviour of the sensors used in the prototype. The results of several analyses done on the data obtained from beamtests at both DESY and CERN, as well as compare these results with simulations done are discussed. This will test both the validity of the simulations as the quality of the beamtest results. The conclusions drawn from the analyses will be used to give a final impression based on all results. A number of suggestions for future improvements to the prototype and detectors derived from the prototype will also be given.

One of the fundamental questions in the phenomenology of Quantum Chromodynamics (QCD) is what the properties of matter are at the extreme densities and temperatures where quarks and gluons are in a new state of matter, the so-called Quark Gluon Plasma (QGP). Collisions of high-energy heavy-ions at the CERN Large Hadron Collider (LHC), allow us to create and study the properties of such a system in the laboratory. Anisotropic flow (vn) is strong evidence for the existence of QGP, and has been described as one of the most important observations measured in the ultra-relativistic heavy-ion collisions. In this thesis, the anisotropic flow of not only charged particles but also identified particles are presented. In addition, the investigations of correlations and fluctuations of both flow angle (symmetry plane) and magnitude were discussed. The main goal of this thesis is to understand the nature of anisotropic flow and its response to the initial geometry of the created system as well as its fluctuations.

The measurement of charge-dependent correlations between positively and negatively charged particles as a function of pseudorapidity and azimuthal angle, known as the \emph{balance functions}, provide insight to the properties of matter created in high-energy collisions. The balance functions are argued to probe the creation time of the particles and are also sensitive to the collective motion of the system. In this thesis, I present the results of the measured balance functions in p--Pb collisions at $\sqrt{s_{NN}} = 5.02~TeV$ obtained with the ALICE detector at the LHC. The results are compared with balance functions measured in pp and Pb--Pb collisions at $\sqrt{s}=7~TeV$ and $\sqrt{s_{NN}} = 2.76~TeV$, respectively. The width of the balance functions in both pseudorapidity and azimuthal angle for non-identified charged particles decreases with increasing multiplicity in all three systems, for particles with low transverse momentum value $(p_{T} < 2~GeV/c)$. For higher values of transverse momentum the balance functions become narrower and exhibit no multiplicity dependence. The experimental findings are compared to different models (PYTHIA8, DPMJET, HIJING and AMPT). The comparison with models seems to indicate that for the Pb--Pb system $\Delta\varphi$ is determined mainly by radial flow, while $\Delta\eta$ is less sensitive. In addition, the color reconnection mechanism emerged as a necessary ingredient in understanding the multiplicity dependence observed in the width of the balance function for small systems.

This thesis addresses the thermalisation of heavy-ion collisions within the context of the AdS/CFT duality. The first part clarifies the numerical set-up and studies the relaxation of far-from-equilibrium modes in homogeneous systems. Less trivially we then study colliding shock waves and uncover a transparent regime where the strongly coupled shocks initially pass right through each other. Furthermore, in this regime the later plasma relaxation is insensitive to the longitudinal profile of the shock, implying in particular a universal rapidity shape at strong coupling and high collision energies. Lastly, we study radial expansion in a boost-invariant set-up, allowing us to find good agreement with head-on collisions performed at the LHC accelerator. As a secondary goal of this thesis, a special effort is made to clearly expose numerical computations by providing commented Mathematica notebooks for most calculations presented. Furthermore, we provide interpolating functions of the geometries computed, which can be of use in other projects.

In relativistic collisions between nuclei, the creation of a strongly interacting medium, called the Quark Gluon Plasma (QGP), is expected. It is expected that such a medium also existed in the early universe just after the Big Bang. The phase transition of interest is where the dense medium of free and unbound quarks and gluons cools down to form the ordinary matter consisting of hadrons. The phase transition from ordinary matter to a QGP is predicted in lattice formulations of Quantum Chromo Dynamics at energy densities well above a critical value of 1 GeV/fm3. At present a QGP might also exist in the dense cores of neutron stars. Properties of the QGP can be probed by high-energy partons interacting with the constituents of the QGP. In this interaction the high-energy parton exchanges momentum with the medium. As a consequence, the parton loses energy by radiating gluons. This process is called jet quenching. High-energy partons are created in the early phase of a heavy-ion collision in hard scattering processes of the constituents of the colliding nucleons. They are a well calibrated probe since the initial parton production rates can be calculated with perturbative QCD. The scattered partons propagate through the medium and fragment into jets of hadrons. This process is expected to be modified in heavy-ion collisions compared to proton-proton collisions due to jet quenching. In this thesis a phenomenological study as well as an experimental observable of jet quenching is presented. A selection of parton energy loss models is studied in a simplified and a realistic medium geometry. Due to different approximations in the parton energy loss implementations of the models, the predictions of the medium energy density in heavy-ion collisions differs significantly. High-pT measurements at RHIC and LHC are compared to model calculations. All models have difficulties describing multiple observables simultaneously. The production of jets in heavy-ion collisions is measured with data collected by ALICE at the LHC at √sNN=2.76 TeV. For this analysis jets are reconstructed from charged particles detected in the central tracking detectors of ALICE. The charged particles are the constituents of the jets and are measured with a high efficiency down to very low transverse momenta (150 MeV/c). The challenge in heavy-ion collisions is to separate the particles originating from the hard parton and from the soft underlying event. The background from soft particle production is determined for each event and subtracted. The remaining influence of underlying event fluctuations is quantified by embedding different probes into heavy-ion data. A strong suppression of inclusive jet production is observed in central events when the energy density of the created plasma is highest. For more peripheral events the jet spectrum is less suppressed. The observed suppression for jets is found to be similar to charged hadrons, which suggests that the radiated energy captured by the jet is limited. The ratio of jets with resolution parameters R=0.2 and R=0.3 is found to be similar in pp and Pb-Pb events, indicating no strong broadening of the radial jet structure.

In relativistic heavy-ion collisions a dense and hot medium is created. It is thought to be the quark-gluon plasma (QGP), a state of matter in which the quarks and gluons, normally confined in hadrons, are (quasi-) free. The QGP is believed to have existed in the first few microseconds after the Big Bang and is conjectured to still exist in the neutron star cores. Anisotropic flow is a measure of the momentum anisotropy of particles created in a heavy-ion collision, a consequence of the initial spatial anisotropy of the created medium. This initial anisotropy is largely a consequence of the ellipsoidal shape of the overlap region between the two colliding nuclei. Higher order anisotropy (triangular, rectangular etc.) is the consequence of fluctuations of the shape of the nuclei. Anisotropic flow is expressed by the coefficients of the harmonic decomposition of the anisotropy of the produced particles. Flow is a unique observable, sensitive to the early properties of the created system. The properties of the QGP like the shear viscosity per degree of freedom and expansion parameters are studied using relativistic hydrodynamical models. The integrated elliptic flow (the second harmonic coefficient) of charged particles in Pb-Pb collisions at the LHC with the energy of 2.76 TeV per nucleon pair measured with the ALICE detector is 30% larger than in Au-Au collisions at the relativistic heavy-ion collider (RHIC) with the energy of 200 GeV per nucleon pair, while the flow as function transverse momentum remains virtually the same. This observation is compatible with the predictions of some hydrodynamical models that account for viscous effects. The rise of the value of integrated flow is attributed to a higher value of the mean transverse momentum at the LHC. The measurement of elliptic flow of identified pions, kaons and anti-protons shows a larger mass splitting than in the RHIC measurement. This effect is associated with a higher expansion velocity of the medium (radial flow) and it provides an explanation for the higher mean transverse momentum at the LHC concluded from the rise of integrated flow. The third harmonic coefficient (triangular flow) of identified particles shows a weak centrality dependence conforming with the expectation that higher order anisotropies are caused by fluctuations of the initial conditions. Qualitative similarities with elliptic flow (like mass ordering) support the idea that the mechanism responsible for the higher order momentum anisotropies of the produced particles is similar to that of elliptic flow. The elliptic and triangular flow data seems to favour hydrodynamical models with a very low value of viscosity, close to the conjectured theoretical minimum; a quantitative description needs more theoretical and experimental research. The transverse energy scaling of flow per constituent quark observed at RHIC is broken for elliptic flow en only holds approximately for triangular flow. The idea that quark coalescence is a dominant hadronization mechanism in Pb-Pb collisions at the LHC is difficult to defend; many different processes probably play a role. Also in this case further theory advancements are needed to describe this system quantitatively.

In this thesis the results are presented of the first measurements of the D*+ meson nuclear modification factor RAA in heavy ion collisions at the Large Hadron Collider (LHC) using the ALICE (A Large Ion Collider Experiment) detector at CERN. These open charmed mesons are a useful tool to investigate the properties of the Quark-Gluon Plasma (QGP) since their parent charm quarks are produced in the early stages of heavy ion collisions, so that they are sensitive to the full formation history of the QGP, and their high mass limits their energy loss in the medium. Baseline measurements of charm production in proton-proton collisions have also been performed, since they are required to form the understanding of charm production and suppression in the QGP. The pT-differential inclusive production yield of prompt D*+ mesons, in the rapidity range |y

The structure of this document is the following: Chapter 2 contains a brief description of the ALICE sub-detectors relevant for this analysis. In chapter 3, the approach to vn of decaying particles by multi-particle correlations and reconstruction of the Qn vectors is discussed. Chapter 4 encloses the description of the reconstruction of candidates and their v2. Chapter 5 is where the measurements are presented and discussed. Final remarks will be given in chapter 6.

The standard model of particle physics describes all known elementary particles and the forces between them. The strong force, which binds quarks inside hadrons and nucleons inside nuclei, is described by the theory of Quantum Chromodynamics. This theory predicts a new state of matter at extreme temperatures and densities: the Quark Gluon plasma. The ALICE experiment at the Large Hadron Collider near Geneva was build to study this QGP by looking at collisions of the most heavy stable ions: lead (Pb) ions. In such collisions one hopes to achieve sufficient energy density for the creation of a QGP. One of the signatures of QGP formation in high energy heavy ion collisions is the presence of collective behaviour in the system formed during the collision. This collectivity manifests itself in a common velocity in all produced particles: a collective flow. The most dominant contribution to collective flow is elliptic flow, which originates from the anisotropic overlap region of the two nuclei in non-central collisions and is visible in the azimuthal distribution of the produced particles. Elliptic flow is related to the equation of state of the system and its degree of thermalisation. The analysis of elliptic flow is complicated by the presence of correlations between particles from other sources, summarised in the term nonflow. Several analysis methods have become available over the years and have been implemented for elliptic flow analysis within the ALICE computing framework. These methods have different sensitivities to these nonflow correlations. Because the centre of mass energy at the LHC is so high, predictions have been made of collective behaviour even in proton-proton collisions. These predictions are very divers and give values between 0 and 0.2 for elliptic flow using different models. To constrain these predictions proton-proton data, recorded with the ALICE experiment at the LHC in the 2010 7 TeV proton-proton run, was studied. In proton-proton collisions large nonflow correlations are certainly present and might mask the elliptic flow correlation. The nonflow correlations have to be suppressed sufficiently such that the elliptic flow signal becomes detectable. Therefor an analysis method was choosen that can suppress nonflow correlations by increasing the separation in pseudorapidity of two subevents. This method is called the scalar product method. How much nonflow is suppressed is shown to depend on the pseudorapidity range of the nonflow. The dependence on the pseudorapidity gap size between the subevents, in 7 TeV proton-proton collisions, points to a strong nonflow component, because the signal decreases with increasing gap size. The corresponding Monte Carlo data set shows the same dependence, while it only includes nonflow correlations. This enforces the conclusion that nonflow is the dominant or the only correlation in 7 TeV proton-proton data at the LHC. The conclusion from this analysis is that elliptic flow in 7 TeV proton-proton collisions with at least 10 particles is less than 0.05. Predictions of a higher elliptic flow for these events can be excluded. To exclude or confirm lower predicted values the nonflow contribution has to be further reduced.

During the 2008 run the Relativistic Heavy Ion Collider (RHIC) at the Brookhaven Nation Laboratiory (BNL), NY, provided high luminosity in both p+p and d+Au collisions at $\sqrt{s_{NN}}=200\mathrm{\,GeV}$. Electromagnetic calorimeter acceptance in STAR was enhanced by the new Forward Meson Spectrometer (FMS), and is now almost contiguous from $-1

The goal of high--energy heavy--ion physics is the full characterization of the quark--gluon plasma (QGP), which is a phase of strongly interacting matter where quarks and gluons are no longer confined in the nucleons and can move freely over longer distances. Such a phase probably existed shortly after the Big Bang, and can be produced in laboratory by heavy--ion collisions at a sufficiently large energy density. This new phase of matter is distinctly different from usual hadronic matter. Several signatures have been proposed to probe the QGP. The focus of the present work is on jet production. Jets are collimated beams of particles emitted in the collision. The case in which one jet is associated with the emission of a single direct photon ($\gamma$--jet event) is of particular interest. Photons go through the medium without interacting. For momentum conservation, the energy of the photon is equal to the energy of the initial jet. On the other side, jet particles interact with the medium, losing energy. The energy loss is an important parameter to characterize the medium density. Due to the high multiplicity in heavy--ion collisions, it is difficult to identify the case of $\gamma$--jet production. There are many sources of photons, the most dominant of which is the decay of neutral mesons (mainly $\pi^0$ and $\eta$). Analyzing the simplest case, that is p+p collisions, gives a necessary baseline measurement for $\gamma$--jet correlation studies in Au+Au collisions. This work focuses on di--hadron correlation, seen as a background study for $\gamma$--jet correlation. The properties of jets in p+p and d+Au collisions are analyzed. In this way, di--jet events are completely characterized, providing a reference frame for the study of $\gamma$--jet events. In the present analysis p+p and d+Au data, collected at the STAR experiment, have been studied. The detectors mainly used are the Barrel Electromagnetic Calorimeter (BEMC) and the Time Projection Chamber (TPC). The BEMC is a lead--scintillator sampling calorimeter located at mid--rapidity ($|\eta| has been evaluated as a function of jet particles momentum. It is possible to study the relations between the two jets of a di--jet event from the parameters describing a single jet shape. The di--jet acoplanarity is measured with an azimuthal particle correlation approach. In this thesis the results for and have been extended to higher associated particle pT compared to previous measurements from other experiments, results obtained with charged hadrons and full jet reconstruction. In the overlap region the present results agree with all the previous ones.

The primary goal of heavy ion physics is the study of the Quark Gluon Plasma (QGP). In this exotic state of matter, the quarks and gluons, which make up ordinary hadronic matter, decouple and behave as free particles. The QGP exists at extremely high temperatures and baryon densities, for example, in the early universe or in the ultra-dense core of neutron stars. The thermal radiation from the quarks in the plasma can provide direct information on the temperature evolution and, hence, on the equation of state of the QGP. To isolate these thermal photons, precise knowledge is required on alternative sources of photon production in hadronic collisions. This was the principle argument to study the production rates of direct photons in p+p and d+Au collisions. In this thesis, we present the first direct photon measurement with the STAR detector at RHIC. The data were obtained from p+p and d+Au collisions at a center-of-mass energy of 200 GeV per nucleon pair. The cross sections of neutral pion and direct photon production in p+p collisions are in agreement with the results from perturbative QCD calculations. The nuclear modification factor of neutral pion production in d+Au collisions is consistent with unity for transverse momenta ranging from 2 GeV/c up to 15 GeV/c, indicating the absence of strong nuclear effects. In general, the techniques and results presented in this thesis constitute a solid baseline for a future analysis of thermal photon production in Au+Au collisions.